Thailand's army-stacked parliament will vote in an impeachment hearing against ousted Prime Minister Yingluck Shinawatra on Friday, testing a fragile calm between the rural poor and the royalist establishment backed by the Bangkok middle class.

A guilty verdict on the charge of dereliction of duty could see Yingluck, who was removed from office for abuse of power in May days before a military coup, banned from politics for five years.

Security near parliament was tight on Thursday, with around 100 soldiers and police stationed around the building. There were no visible protests.

The charges against Yingluck, Thailand's first female prime minister, concern her role in a loss-making rice buying scheme that helped bring her a landslide election win in 2011.

Yingluck defended the scheme in an almost hour-long address on Thursday, and disputed all the charges against her.

"Banning me for five years would be a violation of my basic rights," Yingluck said at the third and final hearing on her case on Thursday at Thailand's National Legislative Assembly (NLA).

"This case that is aimed solely against me has a hidden agenda, it is politically driven."

Yingluck said the rice scheme, which paid farmers above the market rate for their rice, was good for the economy. "It helped those with lower incomes earn more," she said. "Farmers are the backbone of the country."

For Yinluck's opponents, the rice scheme was expensive, wasteful, and symbolic of the populist policies aimed at buying the rural vote.

Yingluck's supporters say the charges are part of a broader campaign by the ruling military junta, known as the National Council for Peace and Order (NCPO), to limit the influence of her powerful family.

Weng Tojirakarn, a former member of Yingluck's government and protest leader, said her supporters may hold "symbolic protests" if she is banned from political office.

The government of coup leader Prime Minister Prayuth Chan-ocha has urged Yingluck's supporters to stay out of the capital Bangkok this week, worried about a repeat of the street violence that has dogged the country in recent years.

Thailand is under martial law and public gatherings are banned. Still, about 20 of Yingluck's supporters gathered outside parliament earlier this month despite government warnings to stay away. Some held red roses and tried to raise pictures of Yingluck until police told them to put them away.

"A large demonstration will be difficult because martial law is in place but there may still be symbolic protests," Weng said.

Lose-lose for junta

Peerasak Porchit, vice president of the NLA, said the impeachment vote would again polarise the country.

"No matter which way it goes, there will be those who agree and those who disagree. It won't please everyone," said Peerasak.

Prayuth said on Monday that he had not ordered the 220 members of the NLA to vote against Yingluck. The NLA was handpicked by the junta and is considered little more than a rubber-stamp parliament tasked with enacting the widespread reforms Prayuth wants to see before calling an election.

Over 100 NLA members are former or currently serving military officers. A decision to ban Yingluck would require three-fifths of the vote to be held on Friday.

If Yingluck is found innocent, there could be a backlash by middle and upper-class Thais, who took to Bangkok's streets a year ago to protest against her family's political grip over the country. Her brother Thaksin Shinawatra, a populist former prime minister, was removed in a 2006 putsch.

The army staged a coup in May following prolonged protests by Bangkok's middle class aimed at ousting Yingluck, saying it needed to restore order after months of unrest.

The impeachment is the latest chapter in 10 years of turbulent politics that has pitted Yingluck and her brother against the royalist-military establishment which sees Thaksin, a former telecommunications tycoon, as a threat.